The present disclosure relates generally to techniques for logging-while-drilling (LWD) and, more particularly, to techniques for determining formation properties using an LWD tool incorporating an electronic radiation generator, such as an electronic X-ray generator.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Measurement of the bulk density and/or lithology of a subterranean formation are essential to programs for downhole logging, whether wireline or logging-while-drilling (LWD). Conventionally, many downhole well-logging tools use emissions of radiation, such as gamma-rays, to determine a property of a subterranean formation such as lithology or density. Once emitted into the formation, the gamma-rays may interact with the formation through Compton scattering, which may attenuate the gamma-rays, and/or the photoelectric effect, through which elements of the formation may absorb the gamma-rays. The degree to which the formation causes the gamma-rays to be Compton scattered and/or to be absorbed via the photoelectric effect may depend respectively on the density and lithology of the formation. That is, formations of various densities and lithologies will cause Compton scattering and absorption via the photoelectric effect in a predictable manner. Thus, by detecting the spectrum and the number of gamma-rays that return to the downhole tool, the density and/or lithology of the formation may be determined.
Conventionally, the formation bulk density measurement has relied on discrete energy radioisotopic gamma-ray sources, predominately 137Cs. In particular, the principle decay of 137Cs to 137Ba results in the emission of monoenergetic gamma-rays of 662 keV. A typically logging-while-drilling (LWD) gamma-ray density tool may include such a radioisotopic source in a drill collar of a borehole assembly. Gamma-ray detectors may be mounted in a tool chassis within the drill collar, which may be surrounded by gamma-ray shielding with openings pointing to windows permissive to gamma-rays in the collar and the stabilizer. The gamma-ray spectral count rate obtained by the gamma-ray detectors may be used to obtain density and photoelectric factor (PEF) corrected for standoff.
The use of radioisotopic sources such as 137Cs may be undesirable in logging-while-drilling (LWD) tools. Radioisotopic sources may require special handling when such sources are inserted into or removed from the tool, shielding may be required for transportation and storage of the radioisotopic sources, and security measures may be needed when transporting and storing the radioisotopic sources. Moreover, additional complications may be associated with the abandonment of radioisotopic sources in a well if the LWD tool becomes stuck and cannot be retrieved.